The formation and casting of metals and metal alloys of various kinds have been conducted for many years using commercial scale operations. For example, continuous twin roll casters, such as those shown in U.S. Pat. Nos. 2,790,216 and 4,054,173 are commonly used. The casters disclosed therein include an opposing pair of water cooled, counter-rotated and generally horizontally oriented casting rolls. Molten metal is routed through a feed system into the nip of the two rolls just prior to the closest approach of the rolls. Typically, the feed system includes an upstream head box and a feed tip nozzle. The metal is directed from the head box, through the feed tip nozzle and into the nip of the rolls. As the metal comes into contact with the water cooled casting rolls, heat is rapidly extracted and the metal begins to solidify. The solid metal is then compressed into a sheet as it passes through the gap between the caster rolls.
Conventional casting machines of this type are typically capable of producing 6 mm thick strips at productivity rates of approximately 1.7 tons/m width/hour. Recently, however, a new generation of casting machines has been developed for high speed, thin strip casting of molten metal. These new generation casters are capable of casting gauges of less than 1 mm. By developing the technology necessary to cast thinner and faster, it is possible to increase productivity and reduce the number of downstream rolling passes necessary. Specifically, this technology allows for great increases in productivity, greater casting capacity in addition to enhanced quality when compared with conventional casting machines.
In order to satisfy the more demanding requirements of this latest generation of casting machines, a need exits for an improved molten metal feed system. The feed systems currently being used on conventional casting machines have not been able to successfully handle the transition to higher production flow requirements. For example, the feed systems currently being used on conventional casting machines tend to produce uneven, and often turbulent flow through the feed tip nozzle when operated at increased speeds. This turbulence is caused by the presence of baffles, or spacers, within the feed tip nozzle. One or more baffles are typically incorporated along the width of the feed tip to help manipulate and direct the flow of molten metal through the tip. The use of such baffles is described in U.S. Pat. Nos. 4,303,181 and 4,641,767. Although this design has proven sufficient for conventional casting machines operating at nominal production rates, at increased speeds the presence of baffles in the feed tip produces eddy currents in the molten metal as it is being routed through the nozzle which in turn cause the flow to be turbulent.
Additionally, the feed systems currently in use with continuous casters tend to produce a large temperature gradient in the molten metal across the width of the strip. Prior to entering the feed tip nozzle, the molten metal travels through an upstream head box. Since the width of the head box is typically significantly less than the width of the feed tip nozzle, an uneven flow of molten metal may reach the feed tip. Specifically, molten metal may begin to flow through the center section of the feed tip nozzle before a sufficient amount of metal is present to begin flowing through the edges of the feed tip nozzle. Consequently, a temperature gradient is produced in the molten metal along the width of the feed tip nozzle where typically the temperature of the molten metal is greatest at the center of the feed tip nozzle. This temperature gradient affects the profile of the cast sheet.
These and other problems have been experienced when the existing feed system designs are used on machines operating in the high speed, thin gauge range. Many of the casting defects (e.g. buckling, starvation, etc.) experienced on the resulting cast sheet are due to these problems associated with the feed system design. Consequently, a need exists for a molten metal feed system for continuous casters capable of handling the more demanding requirements inherent in high speed, thin gauge casting.